Solid hydrogen has a normal melting point at 14.01.degree. K. and liquid hydrogen has a normal boiling point at 20.35.degree. K. One of the properties of hydrogen which sets it apart from other substances is the co-existence of its two different molecular states: para-hydrogen in which the protons have opposing spins, and the more energetic ortho-hydrogen in which the protons have unidirectional spins. The A mixture of hydrogen in these two states at high temperatures is called normal hydrogen. At room temperature, normal hydrogen has a composition of approximately 25% para-hydrogen and 75% ortho-hydrogen. An equilibrium mixture of para- and ortho-hydrogen at any given temperature is called equilibrium hydrogen. At the normal boiling point of hydrogen, equilibrium hydrogen has a composition of about 99.79% para-hydrogen and 0.21% ortho-hydrogen, practically all para-hydrogen. Conversion of a hydrogen mixture from a high para-hydrogen composition to equilibrium hydrogen at any temperature is an endothermic reaction. Conversion is also extremely slow, consuming hundreds of hours. Consequently, vapor created during evaporation of a mass of solid equilibrium hydrogen retains its almost entirely para-hydrogen composition long after the vapor has reached room temperature.
Sublimation of solid hydrogen may be used as a source of cooling. Additional cooling may be obtained at temperatures higher than the triple point of hydrogen by warming the hydrogen vapor as heat is absorbed.